Phasing system



Nov. 30, w48. c. TowNsEND PHAS ING SYSTEM 2 ShetS-Sheet l Filed June 22, 1946 CHARLES L. lTowNsEND ATTORNEY Patented Nov. 30, 1948 UNITED STATES PATENT OFFICE PHASING SYSTEM Charles L. Townsend, Yonkers, N. Y., assignor to Radio Corporation of America, a corporation of Delaware l Application June 22, 1946, Serial No. 678,547

6 Claims.

image area from which the video signals are developed.

According to present television standards, the image eld repetition frequency is at the rate of 60 elds forming 30 double interlaced frames per second. In film projection the lm is ordinarily so advanced that it passes through the lm gate in such a way as to be projected at a rate of 24 frames per second. The 30 frame projection rate is preferably maintained in television in order that the movement of the subject matter contained on the film appear in an optimum natural state to an observer of the reproduced image, while the 24 frame per second projection rate is customarily used in normal lm projection and particularly so because of the sound signals to be removed from the sound track.

In film projection television systems employing a storage type camera tube of the iconoscope type, for instance, the mosaic, orV lightresponsive electrode of the camera tube upon which the optical image is projected, is scanned by a cathode ray beam in substantially complete darkness, that is, during the period between which successive light ashes of the film image are impressed on the mosaic, The general system is described, for instance by the United States Patent granted to R. D. Kell, Patent No. 2,166,214, on July 18, 1939, so that further detailed reference to the system is unnecessary herein. This scanning operation removes the electrostatic charge image which is built up on the mosaic as a result of the image light impressed thereon. The scanning of the mosaic by the electron beam produces the video signals. The next succeeding electrostatic charge image is built up by a very short 2 back or return trace period of the cathode ra scanning beam, it is necessary that the film frame areas be exposed to the camera tube mosaic at a rate which coincides with the slower scanning operation on the camera tube. It is further necessary that a suitable shutter be positioned along the optical axis of the film projector, and that the operation of this shutter `be so synchronized with both the lm movement and the camera tube beam deflection generators that the image of the lm frame within the film gate is projected upon the camera tube mosaic during the beam snap-A back o1' return in the slow portion of the scanning cycle Where no video signal output is being developed.

The above requirements are customarily met by supplying alternating current to a, synchronous film driving motor, the frequency of this alternating current having a predetermined relationship to the slower (vertical) scanning beam dcection in the camera tube as controlled by the beam deflection generator. The synchronous driving motor is mechanically connected both to the lm moving device and to the shutter mechau nism, so that the resulting operations of scanning, lm movement, and mosaic exposure are properly coordinated.

As above brought out, and as shown by the Kell patent, supra, an image of a stationary iilm area or frame should normally be projected on the light-responsive mosaic electrode of the camera tube only during all or a, portion of each vertical blanking interval. However, with the type of synchronous driving motor frequently employed, it is possible for this motor to lock into the power supply, when the system is first placed in operation, at either of two positions which are 180 (electrically) apart. In other words, it is possible ior the synchronous driving motor to be 180 out-of-phase with the scanning beam deflection as controlled by the deflection generators which are customarily locked to the power supply frequency.

The result of such an out-of-phase condition is that the shutter opens to permit the projection of a light image of the iilm frame in the lm gate upon the camera tube mosaic not during the vertical blanking period, as in the normal manner,

but instead during approximately the mid-portion of the scanning period. This causes 'a horizontal bar of light to appear across the center vof the reproduced image at monitoring or receiving points, as will be pointed out in more detail later. y

It has heretofore been the practice to employ According to a feature of the present invention, y

correct phasing for a projector motor as used intelevision systems is obtained automatically. This K.

is accomplished, in the preferred embodiment described, by utilizing the light projected on the mosaic of the camera tube each time thatY the shutter is opened to produce a series of ,controlv pulses. This control pulse series is superimposed in opposite phase on pulses produced during, and extending ink each direction beyond the limits of thevertical blanking periods. If a control pulse anda blanking interval `pulse occur simultaneously, the controly pulse is cut off by the blanking interval pulse. However, if ythe phasing of the projector motor is incorrect so that the control pulse occurs during some portion of the scanning interval, then the control pulse is available to initiate the operation'of a relay which interrupts momentarily the power supply to theA projector motor in thesame lmanner as the manually-controlled relay mentioned above.

One object of the present invention, therefore,

is to provide an automatic motor and shutter phasing circuit for lm projection television systems. i y

Another object of the invention is to provide` a film projection television systemin which acontrol variation is obtained for each Vprojection of a film image on the light-responsive electrode of the camera tube, and means for comparing the phase of such control variation with that of pulses produced during the vertical blanking intervals, whereby a departure of said control variation from a predetermined phase relationship with the blanking interval pulsesacts to bring about a re-establishment of the saidA predetermined phase relationship.

Other` objects and advantages will be apparent, from the following description of a., preferred form of the invention and from the drawings,. in which: 4

Fig. 1 is a partly schematic showing of a film projection television transmitting., systemin iac- Vcordance with the present invention;

Fig. 2a represents graphically the ratio between the mosaic exposure time and. the scanning time in the television system of Fig. 1;

Fig. 2b represents graphically one possible time',

ratio between stationary and` moving periods of the motion picture lm which is vnecessary in order that the purposes of the present invention may be accomplished; u

Fig.. 3 represents a plan view of the disc shutter:

employed in the system of Fig. 1; and l Fig. 4 illustrates the light bar whichmayappear in the reproduced' image when the vphasing ofthe projector motor of Fig. 1 is incorrect. Referring irst to Fig.-1, there is shown a reel I0 from which the lm to be shown bytelevision is withdrawn. The lmv [2v-iis passed. over an idler roller I4 and through a lm gate I6. The lm I2 is drawn through the gate I6 in an intermittent manner.,y asdeterminedgbythe intermittent rotation of 'the'sprocket wheel I8. VAs any supplyzknown intermittent device may be used, as is also indicated by the mentioned Kell patent, the showing herein is completely diagrammatic and not specific. 'I'he film proceeds over an additional idler roller 20 and is Wound up on the take-up reel 22,

Fig. 1 alsoshows a television camera tube 24 (illustratively shown as anficonoscope; although any other form of storage tube, such as the image viconoscope the orthicon or image orthicon may also be used) including a light-responsive mosaic or electrode 26 with which is associated a signal plate 28. Within the` camera tube 24 is an electrorigun structure 30 for producing a beam of electrons, this beam of electrons being deflected in mutually perpendicular (usually horizontal andvertic'al) directions by means of the magneticfields produced by the deflecting coils 32 and 34 respectively. These coils 32 and 34, by reason of their cyclically varying energization, cause the cathode ray beam to kscan systematically. the light-responsive mosaic electrode 26. Thevideo signal output from the television camera tube` 24 is'derived from the Vsignal* plate 28 V38; in accordance 'with 'known `television practice,

utilizes the video signals' from tube 36 andsynchronizing signals from a master oscillator (to be later described) to modulate a carrier wave in order that the signal energy maybe transmitted by means of an antennaarrangement 40, or such signals'may be' supplied in anyY desired manner to any. other form of communication channel,

such as aV coaxial cable.

W'hen'an optical image is projected uponthe light-responsive electrodel 26, electrostatic charges vare produced. thereonAV in accordance with the intensity of the optical image. As the cathode ray'b'ealriemitted from the electrongun 30 scans the, light responsive electrode 26, the charge 'image'is removed; and 'aseries of video signals Vaccordii'igly developed: After` the charge limage has been entirely removed from the light-responsive electroda another optical. image is projected 'thereon' for a short space of'time,V during which thecathode ray scanning beamv is blanked or suppressed andreturned in 'a vertical direction to its starting `point for the next successive scanning cycle. During the scanning of the light-responsive electrode, no lm image lightis permitted to' strike the electrode. v

For projecting optical. images from `the motion picturefilm frames `ontol the mosaic 26;` a source of light 42-is provided, with which is associated a reflector IM.y Thelig'ht from thesource 42'is directed upon that portion of the lm I2 Whichis present in thel-m. gate I6 lby means of a condensing lens 46'. The lightwhich is pormitted topassthrough the lm is-.rfocused `upon the `light-.respensive electrode ormosaic 26 by means, of.. the projection or focusing. lens system 48.5. .A diseshutter isA positioned along the optical axis of the projectorand is generallylocated between the focusing lens 48 and the cam- Fig, and normally comprises an opaque disc having' an aperture or slot 52 on or near the periphery thereof.

Since the television eld scannings of the image are at the rate of 60 per sec-ond, the speed of the shutter 50 is so chosen that the light images are projected on the light-responsive electrode 26 at the rate of 60 images per second, these images being actually projected on the lightresponsive electrode 20 only during the time period between successive transmissions of the separate television iields, that is, between each successive field-scanning operation. In Fig. 2a is shown a representation of several television fields. These fields are indicated as a function of time, and, as represented in the drawing, each field cycle occupies 140 of a second. Of this cycle, as explained above, not over 10 lper cent, or 174500 of a second, is utilized for the vertical return of thev cathode ray scanning beam. Th-is return period coincides with the time that the light image from the iilm I2 is projected on the mosaic 26, or, in other words, the mosaic exposure time. Between each mosaic exposure time is a period of at least of a second, which is actually the useful field-scanning time of the mosaic 26. The general film advance method is also set forth by reference to United Stat-es patent of S. W. Seeley, No. 2,303,960, granted December 1, 1942.

Fig. 2b is related to Fig. 2a and shows the movement which may be imparted to the nlm I2 in order that 60 television fields may be transmitted from a film operating at 24 vframes per second. One iilm frame cycle, as indicated in the drawing, occupies a time period of 1/4 of a second, the film being moved during a portion of this cycle which corresponds approximately to 1/150 of a second.

Since the exposure of the camera tube mosaic 20 must occur while the film I2 is stationary in the gate I6, a 4properly synchronized relationship must exist between the motion of the film I2 and the scanning operation of the camera tube 24. The means for obtaining this relationship includes a synchronous motor 54 on the shaft of which is mounted the disc shutter 50. Since this disc shutter 50, as shown in Fig. 3, is provided with a single aperture 52, the motor 54 is arranged to operate at a rate of 3,600 R. P. M. As shown in Fig. 3, the length of time consumed during one revolution of the disc 50 will then be 1/50 of a second, and since the vertical return time of the -cathode ray scanning beam occupies a `period not in excess of 1/10 of a complete field cycle, the aperture 52 has an arcuate dimension corresponding in time toan interval not greater than 1/;00 of a second.

In order that the film I2 may be moved in synchron-ism with the operation of the camera tube 24 and in synchronism with the rotation of the shutter disc 50, the film driving sprocket wheel I8 is mechanically connected to the motor 54 by means of a gear box 58. This -gear box 56 contains gearing such that the sprocket wheel I8 will move the i'llm I2 at a constant rate of 24 frames per second and also such that a regular intermittent motion will be imparted to the sprocket wheel I3.

Power for the projector apparatus and for the transmitting system is supplied from the power line terminals 58 to which is connected a master oscillator and synchronizing device 60. This master oscillator 60 supplies impulses at the rate of 60 per second which are -fed to the 60 cycle vertical or field deiiection generator 62,

- which operates through the deecting coil 34 to cause the cathode ray scanning beam' to be deflected vertically at the rate of 60 deiiectlons per second. The waveform of the energy supplied to the deection coils 34 is generally of sawtooth configuration or some modification thereof. The master oscillator 60 also supplies impulses at line frequency to the horizontal, or line, deflection generator 64, which operates to generate the proper waveform for energizing the horizontal deiiection coil 32. The latter imparts the proper horizontal deection to the cathode ray scanning beamin order to scan each line of the mosaic 26.

Energy is also supplied from the power line input portion of the master oscillator 60 by means ofyconductors 66 to the synchronous motor 5d, ythe frequency of this energy thus being in fixed time relation with respect to the impulses which are supplied to the line and field deiiection generators 64 and 62, respectively.

Since the television receiver must be acc-urately synchronized with the operation of the television transmitting system, synchronizing impulses are transmitted with the video signals. Accordingly, impulses from the master oscillator and synchronizing device 60 are supplied to the amplifier, mixer, and transmitter 38 by means of conductors 68. These synchronizing impulses are generally transmitted at the end of each line-scanning interval and at the end of each field-scanning interval, that is, during the horizontal and vertical retrace periods of the cathode ray scanning beam.

The motor 54, as above stated, receives energy from the power line input portion of the master oscillator 60 over the conductors 66. Since the master oscillator 60 supplies energyto the vertical deection generator 62, it is evident that the motor 54 may have its operation synchronized with the field-scanning operation of the camera tube 24. When the disc shutter 50 is properly phased with the deflection of the cathode ray scanning beam, then the aperture 52 in the disc 50 is in alignment with the optical axis of the projector only during the time that the cathode ray scanning beam is being returned in a vertical direction to begin another field-scanning cycle. In other words, the nlm image is projected through the aperture 52 onto mosaic 26 only during,r the vertical blanking interval.

However, the synchronous motor 54 is so designed that it may lock into the power supplied by the master oscillator 60 at either of two positions which are apart. In one of these positions, the aperture 52 in the disc shutter 5U is aligned with the optical axis of the projector during the vertical blanking interval. In the other of these positions, the aperture 52 is 180 away from alignment with such optical axis during vertical blanking. The result of this latter condition is that the aperture 52 reaches alignment with the optical axis of the projector not during vertical blanking, but instead approximately at the mid-point of the iield scanning interval. This momentary exposure of the mosaic 26 during the scanning interval causes a light bar to appear across the center of the reproduced image, as shown in Fig. 4.

As previously stated, it has been customary to overcome a condition ofthis nature by utilizing a manually-operated phase-controlling device, which may constitute a relay in series with the motor 54. This manual phase control, as shown in Fig. l, comprises a pair of normally-closed relay contacts 'IIJ ywhich'are adapted to be opened caused to discrete photo-emissive particles.

flammes w-fon the manualclosing of a switch-12. Upon the i closing of switch l2,- the contacts 1G areidisengaged to cut oi .the power supplied over conductors 66 to the motcr'ii, and the contacts l are remain .disengaged (either `by the switch l2 being held open manually or by a timedelay device on the relay itself) until the motor 54 has slipped through approximately a half rev- .olution to thereby attain a proper phase relationship with the deflection generator 62.

According to a feature of the present invention, this phasing operation of the motor 54 is carried out automatically by means which will now be described. The mosaic, or light-responsive electrode, 28 of tion and has on one oi its surfaces a number of When light from the source t3 impinges on the mosaic `28, electrons are released therefrom, and each photoemissive particleof the mosaic acquires a charge the intensity of the illumi- According `to which depends upon nation received hy that particle.

, the present invention, the electrons which are released froni the mosaic (when the latter is illuminated at the time the aperture 52 of disc 59 is in alignment with the optical axis of the projector) are in part collected by a metallic element 14 on the Vinner wall of the camera tube 2d., This element lll, consisting of a silver evaporator used in the manufacture of the camera tube but otherwise previously unused, is connected to a source ofl positive ypotential (not shown) through a resistor i6. Hence, when the shutter 5G opens and exposes the mosaic 2t to light from source 42 for a short period of time, the released electrons are collected by the element "Ml during the exposure period and flow through resistor 1B to develop across this resistor a series of pulses which may have a wave-form such as indicated by the reference numeral 71S.

A transformer SS has its primary winding connected to receive alternating current from the .master oscillator tt, the frequency of this alternating current being equal to the frequency of the impulses supplied by the master oscillator Sil to the Vertical deection generator 62. The output of transformer 89 (which is of substantially sine wave configuration) is applied through a phaseshifting network 82 to the control electrode of an ampliiier and clipper tube 84. The network 82 permits the introduction of any shift in the phase of the wave supplied by the oscillator 60 which may be required for proper operation of the system.

Tube Sli operates to clip the sine wave output of transformer 8l) in the manner indicated by the reference numeral 86. The bias on tube 84, and hence the clipping level, may be controlled by varying a potentiometer 88, the latter being preferably adjusted to remove substantially one entire portion of each cycle of the sine wave output of transformer 8l). Thus each cycle of the wave 86 includes a ilat voltage base portion Si).

When the phasing of motor 54 is correct, the pulses 18 which are produced each time that the shutter disc 50 opens to emit light to the mosaic 26 will coincide in time with the negative excursions of the clipped wave E6. However, when the phasing of motor is incorrect, to thereby result in the placing or a light bar across the center of the image raster as shown in Fig. 4, then the occurrence of the pulses 78 will coincide in time with the production of the flat voltage bases 80.

The` pulses 'i8 developed across resistor 'I6 are camera tube 2xt is of known construci applied to the control grid. of an. amplier tube consisting of the clipped .l bythe wave 86 is mixed withthe inverted pulses '58, as shown bythe waveform S4. It will be. un-

derstood that the solid-line portion of the WaveformV 94 represents: an out-of-phase condition of the motor54, or, in other words, shows the clipped f sine wave output 86 of the amplifier` tube B4 combined with the inverted pulses 18 (produced during the exposure time of mosaic 26) in such a manner that the pulses 18 in effect sit up on the `hat voltage bases .90.

It will be appreciated that, when the motor 54 isoperating in correct phase, the pulsesfl will be displaced a half-cycle in time from their position shown in solid lines in wavellt, or, in other ",vords, the positive pulses 'i8 will occur simultaneously with the negative excursions of `the wave in the manner shown by the broken lines. The peak amplitude of the pulses 18 under the latter condition, therefore, will not .extend appreciably if at all above the flat voltage base portions 53.

This-composite wav-e'Q/Lconsisting of the combined waves irland 8G, is applied to the anode oi a diode clipper tube 96. The cathode of clipper tube is connected to the control electrode ci a gas-illed discharge tube A Q3. Tube .96 operates to clip oil that portion of the composite wave lwhich liesbelow the at voltage base 98. lThis clipping level may be adjusted by varying the positive potential onthecathode of tube 93 through adjustment of potentiometer l.

x mosaic '35 isexposed during thevertical blanking interval, then the pulses 78 will occur during the vnegative excursions of the clipped sine wave 86,

and will not entend appreciably if at all above tiri-e bases. Thus, an in-phase condition of the motor 5ft 'nasbeen represented inthe `waveform Elli by an illustration of the pulses A'i8 in dotted outline. However, when the phase of the motor Ilil is incorrect, then the pulses x18, as above stated, will sit up on the voltage bases 9D, as shown in solid lines, and will be applied as a positive voltage to the control electrode of the gas-filled triodei 93 following .the clipping action of tube 95.

The anode of the gas-filled tube.98 is connected to a source oi positive. potential through therelay winding lZ and an adjustable resistor ill. The upper end of resistor |64 (in the drawing) 4is connected to ground through a charging condenser i. Tube 9B- has its control electrode biased from a source o negative potential (not shown) through potentiometer H18. By varying the biasagplled to the control electrode of tube liti, the firing point of the tube may be altered in any desired manner.

The two normally-closed contacts H0 associated with the relay winding l2A are connected in series with the two normally-closed contacts 'Hl of themanually-controlled relay between the .motor 15d and the master oscillator 50, or, in `other words, the moto1254, the contacts l), and vthe contacts iiD-areeiectively in series relation.

Assuming that vthe tube f. 98. is non-conductive, condenserlvwillibe Acharged from thesource of positive potential through resistor |04. This places a positive potential on the anode of tube 98. As long as no positive voltage pulse is receved on the control electrode of tube 98, the latter will remain non-conductive due to its bias through potentiometer |08. However, when one of the pulses 'i8 is received on the control electrode of tube 98 due to the fact that the motor 54 is operating in incorrect phase, then tube 98 will become conductive, and current will ilow through the tube and also through the relay winding 92. The relay contact H0 will then open to interrupt the power supply to the motor 54. As above brought out, this momentary interruption in the power supply to motor 54 will usually enable the motor 54 to slipsumciently to obtain correct phasing.

The adjustment of resistor 104 determines the charging rate of condenser H36, or, in other words, the frequency at which the gas-,filled triode S8 is rendered conductive when positive pulses continue to be supplied to its control electrode. In practice, it has been found that an operative interval of approximately one second is satisfactory.

While the manual phase control, including the relay contacts lil and the manually-operatedv switch l2, has been illustrated in order to provide a basis for describing the operation of the present invention, it will be appreciated that the v automatic phasing means set forth herein eliminates the necessity for using such a manuallycontrolled device, and in practice these components will normally be omitted.

While the pulses 18 have been shown as being obtained from the silver evaporator element 14 on the camera tube 24, it will be obvious that these pulses may be derived in a number of other ways such, for example, as by the diversion of a portion of the light passing through the aperture 52 onto a suitable photocell or other lightresponsive medium.

Having thus described my invention, I claim:

1. In a television transmitter for the transmission of images, said transmitter including a motion picture lm, a camera tube having a mosaic electrode, a motion picture projector including mechanism for intermittently moving said iilm into position whereby optical images of individual film frame areas are projected successively from said hlm upon said mosaic electrode thereby to develop on said mosaic electrode electrostatic charges, proportional to the intensities of light and shadow on the iilm frame projected, a synchronous motor for driving said intermittent mechanism, means in said camera tube for developing a cathode ray scanning beam to scan said mosaic electrode and means to cause the said beam to scan the mosaic thereby to produce video output signals, the combination of means responsive to the successive reception of projected optical images on said mosaic electrode for deriving a rst voltage variation, means for deriving a second voltage variation having a fixed time relation with the deflection of said cathode ray scanning beam, means for combining said iirst and second voltage variations so that a control eiect is produced when said variations have a predetermined phase relationship indicating a departure from phase identity between the synchro-nous motor and the deiiection of said cathode ray scanning beam, and means to effect a time controlled interruption of the current supplied to said synchronous driving motor under the 10 control of the developed control effect thereby to bring about a change in phasing of said motor with respect to said beam deflection and establish phase identity therebetween.

2..Apparatus according to claim 1, in which vsaid means responsive to the successive reception of projected optical images on said mosaic electrode for deriving a rst voltage variation includes means for collecting the electrons released by said mosaic electrode due to the impingement thereon of said optical images.

r3. Apparatus according to claim l, in -which said means to effect a time controlled interruption oi the current supplied to said synchronous driving motor includes a normally-closed relay in series with a grid-controlled electron discharge device, said control effect being applied to the grid of said electron discharge device to render said device conductive and thereby control the openingl of said normally-closed relay as a function'thereof.

4. In a television transmitter for the transmission of images, said transmitter including moving picture hlm, a camera tube having a mosaic electrode, a motion picture projector including mechanism for intermittently moving said iilm into position whereby optical images of individual lm frame areas are projected successively from said film upon said mosaic electrode thereby to develop on said mosaic electrode electrostatic charges representative of the intensities of light and shadow in the film frame projected, a synchronous motor for driving said intermittent mechanism, means in said camera tube for developing a cathode ray scanning beam to scan said mosaic electrode and means to cause the said beam to scan the mosaic in mutually perpendicular directions and thereby produce video output signals as a result of the neutraliza tion by said scanning beam of said electrostatic charges, the combination of a normally-closed circuit breaker connected in series with said synchronous motor, means responsive to the successive reception of projected optical images by the mosaic electrode of said camera tube for deriving a series of voltage pulses, means for deriving a sine Wave having a frequency equal to that at which said cathode ray scanning beam is delected in one of its mutually perpendicular directions, means for clipping said sine wave at a substantially predetermined level so that each cycle of said wave will include a flat base portion, means for combining said series of voltage pulses with said clipped sine wave so that each pulse of said series will coincide in time with a corresponding flat base portion of said. sine wave when said synchronous motor is out oi phase with the deiiection of said cathode ray scanning beam in the said one direction, means for removing the remaining portion of said clipped sine wave from said combined Wave so as to leave only the said voltage pulses, and means for applying the voltage pulses thus remaining to control the opening of said normally-closed circuit breaker and thereby interrupt the current supply to said synchronous motor.

5. Apparatus according to claim 4, in which said means to control the opening of said normally-closed circuit breaker includes a gas-nlled discharge tube and a condenser in series with said circuit breaker, a resistor, means for charging said condenser through said resistor from a source of potential, and means for varying the value of said resistor to control the rate of charging of said condenser and hence the anode'poten-f tialof said gas-filled discharge'tube.

6. "In a television transmitter', for the"tran'smis sion of images, said 'transmitter including' a movfingjpicture film', a cameratube having-amosaic electrode; a moti'onpictureprojector including mechanism' fore intermittently zmoving 'saidA li'n" oping'and deflecting'a cathode'ray scanning beam" to scan said mosaic `electrode and thereby pro-A duce'video output sign-als as a'result' of'theneutralizationby'sai'd scanning beam of s'aid electrostatic charge' images, thecombination uffa nor-` many-closed circuit breaker" c'onne'ctedin series' with said synchronous' motorymeans* responsive to the successive reception of"projecte`d' voptical images bythe mosaic electrode of said camera tube for deriving a rst voltage-variation, means for deriving a second voltage variation having a frequency substantially equal to the field-scanning' frequency aat vwhich saidl cathode ray scan- 'ningbeam is deflected-means for comparing the phase `of theitwo voltage variations, and means forapplying'the control effect produced as a result-of-a substantially predetermined phase relationship `ofi'th'e two said voltage variations to ycontrolthe opening of the said normallyclosed circuit breaker.`

CHARLES L. TOW N SEND.

REFERENCES CITED' The following references are of record in the le of this patent:.

UNITEDV STATES i PATENTS Name Date 2,165,778 Beers July 11, 1939 

